Wind turbine with parallel converters utilizing a plurality of isolated generator windings

ABSTRACT

A system and method are provided to isolate outputs of parallel converter threads of a power system converter on a generator side of a wind turbine generator by utilizing isolated power windings on the wind turbine generator. Such isolation eliminates the circulating common mode current between the parallel converters of the wind turbine system and eliminates the need for a common mode inductor. System reliability is enhanced and total system cost is reduced.

BACKGROUND OF THE INVENTION

The invention relates generally to wind turbine generators and morespecifically to a method of interconnecting parallel power convertersfor the wind turbine generators to eliminate a common mode current,which circulates between the parallel power converters.

Generally, wind turbines use the wind to generate electricity. The windturns multiple blades connected to a rotor. The spin of the bladescaused by the wind spins a shaft of the rotor, which connects to agenerator that generates electricity. Specifically, the rotor is mountedwithin a housing or nacelle, which is positioned on top of a truss ortubular tower, which may be as high as about 100 meters. Utility gradewind turbines (e.g., wind turbines designed to provide electrical powerto a utility grid) can have large rotors (e.g., 30 or more meters indiameter). Blades on these rotors transform wind energy into arotational torque or force that drives one or more generators,rotationally coupled to the rotor through a gearbox. The gearbox may beused to step up the inherently low rotational speed of the turbine rotorfor the generator to efficiently convert mechanical energy to electricalenergy, which is provided to a utility grid. Some turbines utilizegenerators that are directly coupled to the rotor without using agearbox. Various types of generators may be used in these wind turbines.

Many devices, such as wind turbines, include power converter systems. Apower converter system is typically used to convert an input voltage,which may be fixed frequency alternating current, variable frequencyalternating current, or direct current, to a desired output frequencyand voltage level. A converter system usually includes several powersemiconductor switches such as insulated gate bipolar transistors(IGBTs), integrated gate commutated thyristors (IGCTs or GCTs), or metaloxide semiconductor field effect transistors (MOSFETs) that are switchedat certain frequencies to generate the desired converter output voltageand frequency. The converter output voltage is then provided to variousloads. Loads, as used herein, are intended to broadly include motors,power grids, and resistive loads, for example.

FIG. 1 is a block diagram of a typical power system coupled to a windturbine with synchronous wound-field or permanent magnet generator andimplemented according to one aspect of the invention. The power system10 is adapted to provide output power to grid 21. A wind turbine 12 isconfigured for converting wind energy to mechanical energy. The windturbine is coupled through a gear box 19 to generator 14 oralternatively coupled directly to generator 14. Wind energy is capturedby the rotation of the wind turbine's blades, and generator 14 isconfigured by a power converter system 20 controlled by convertercontrol system 24 for generating a variable frequency input power. Thepower is transformed to appropriate voltage by one or more transformers22 and supplied to the power grid 21.

To accommodate the need for greater power from windfarms, individualwind turbine generators are increasingly being provided with higherpower output capability. To accommodate the higher power output from thewind turbine generators, some wind turbine systems are provided withmultiple parallel converters (also known as converter threads). Multipleparallel converters may also provide an advantage in wind converters dueto the desire for high availability and low distortion

Typically, power converter systems use multiple power converter bridgesin parallel with gating control to expand power-handling capability. Inwind turbine applications, a power converter bridge usually refers to athree-phase converter circuit with six power switches. In order to meetboth grid side and machine side power quality requirements, such systemsgenerally use very large and costly filters to smooth out pulse widthmodulated waveforms. Such systems sometimes cause overheating of thegenerator and/or transformers and other distortion-sensitive equipmentdue to high harmonic components, when the large and costly filters areminimized.

FIG. 2 is a block diagram of a typical power system employing multipleparallel converters. Power system 10 is configured for supplying powerto a load 21. A generator source 14 is configured to generate an ACinput power. The AC input power is provided to power converter system20. The power converter system 20 comprises converters 20-1 through20-N. The converters are coupled in parallel and configured to receivethe AC input power from the generator source 14. The power convertersystem 20 is configured to convert the AC input power to an AC outputpower. The AC output power is provided to load 21. Loads may includemotors, power grids, and resistive loads, for example. Although gridsare traditionally suppliers of power, in most wind turbine systemembodiments, wind turbine power is supplied to a utility grid, whichacts as a load.

The plurality of multiple parallel converters 20-1 to 20-N, each one ofwhich (also called threads) has a fraction of the net system rating.These converter threads are tied together on both the input and outputends to form a net current/power rating on both the input and outputthat is directly related to the number of converter threads in parallel.Typically, one side of the converter is connected to a common powersource (for example the grid) and the other to a plant (for example agenerator). The circuit connecting the converter to the power grid willusually be referenced to ground. For cost and size reasons, each threadis connected to a common point on the grid and the plant with conductorsthat are sized in accordance with the rating of each thread and not thesystem rating.

Converter control system 24 is configured to provide control signals forthe operation of the power converter system 20. The converter controlsystem is coupled to the converter system and is configured to drive theconverter system according to predesignated switching patterns. Thepredesignated switching patterns provided by the converter controlsystem may provide for synchronous gating of the multiple parallelconverters or may provide an interleaved manner of control for eachconverter thread with phase displaced gating signals to reduce overallswitching harmonic components due to cancellation of phase shiftedswitching waveforms.

FIG. 3 is a block diagram of a typical thread one thread of a powerconverter system. Wind turbine embodiments, for example, typicallycomprise three-phase power converter systems. Converter 20-1 representsone thread of power converter system 20. Converter 20-1 includesgenerator converter bridge 30 for AC-DC conversion, DC link 35, and loadconverter bridge 40 for DC-AC conversion at a suitable voltage andfrequency. Generator converter bridge 30 may be implemented using sixsemiconductor power switches 45. Similarly, load side bridge 40 may beimplemented using six semiconductor power switches 45. Generator sidechokes 50 and load side chokes 55 may be sized to enable eithernon-interleaved or interleaved gating

Switching of power semiconductors in the converter threads causes adifference in voltage between the parallel converters, which creates acommon mode current that flows between the converter threads, evenwithout having a ground fault on the system. The common mode currentwill flow in a circular loop between the power converter threads, butnot have any impact on the net current in either the grid or the plant.Common mode chokes 60 suppresses the high frequency (switching frequencyrange) common mode cross current that links both generator sideconverters and the load side converters.

FIG. 4 illustrates common mode current flow in a power system converterwith n-paralleled converter threads (20-1 to 20-n) connected to a grid21 and to a wind turbine generator 14. For example, it is possible thata current can flow into thread T1_L_Ia 110 and out T1_G_Ia 115 andreturn through thread T2_G_Ia 120 and T2_L_Ia 125. There are manycombinations of loops for such current that will not affect the netcurrent. However, these common mode currents, as well as normal modecirculating currents, force converter switching devices and othercomponents to operate closer to thermal limits. Further, these commonmode currents may cause a direct error in the measurement of groundfault currents of that loop, thereby making fault detection moredifficult. Large common mode inductors are required to limit the amountof circulating common mode current between the converters, as well as,large normal mode reactors to limit circulating normal mode currentwhere phase shifting is utilized to reduce net distortion.

Accordingly, there is a need to provide a structure and method forinterconnecting the wind turbine generator windings with the powerconverter in a manner so as to reduce or eliminate the common modecurrent that flows between the parallel power converters, without theneed for common mode inductors coupled to a capability to phase shiftmultiple thread converters to reduce the need for bulky filters.

BRIEF DESCRIPTION OF THE INVENTION

Recently, wind turbines have received increased attention as anenvironmentally safe and relatively inexpensive alternative energysource. With this growing interest, considerable efforts have been madeto develop wind turbines that are reliable and efficient. This inventionrelates to a specific method of interconnecting power converters in windturbines, which allows optimization of system cost and reliability byeliminating the common mode current that circulates between parallelpower converters.

According to a first aspect of the present invention, a power system forproviding an output power to a load is provided. The power systemincludes a generator configured to generate an alternating current inputpower to a power converter system. The power converter system is coupledto the generator and interconnected to generate an output power andprovide the output power to the load, wherein the converter systemincludes a plurality of parallel converter threads. A converter controlsystem is coupled to the power converter system and configured to drivethe converter system to reduce harmonic components in the output poweror the alternating current input power. Isolation means for the inputpower to the converter from the generator are adapted to prevent commonmode current circulation between the parallel converter threads.

According to a second aspect of the present invention, a method isprovided for eliminating common mode currents in a power system thatsupplies a power output to a load from a power converter system,including a plurality of parallel converter threads. The method includesoperating the generator according to a controller to supply ac power topower system converter; supplying the ac power to the power systemconverter through a plurality of sets of isolated power generatingwindings of the generator, wherein one set of the isolated powergenerator windings of the generator feeds a corresponding converterthread of the plurality of converter threads. The method furtherincludes operating the plurality of converter threads according to acontroller for the power converter system; adapted to drive the powerconverter system to reduce harmonic components in the power output orthe alternating current input power.

According to a third aspect of the present invention, a wind turbinepower system is adapted to prevent common mode current circulation amongpower system converter threads when providing output power to anelectrical power grid. The wind turbine power system includes a windturbine generator controlled by a wind turbine controller configured togenerate an alternating current input power to a power converter system.The power converter system is coupled to the wind turbine generator andinterconnected to generate an output power and provide the output powerto the load. The power converter system includes a plurality of theparallel converter threads. A converter control system is coupled to thepower converter system and configured to drive the parallel converterthreads to reduce harmonic components in the output power or thealternating current input power. The isolation means for the input powerto the converter from the wind turbine generator is adapted to preventcommon mode current circulation between the parallel converter threads.A plurality of sets of isolated power windings on the wind turbinegenerator is provided. Each set of the plurality of sets of isolatedpower windings on the generator interconnects with only one of acorresponding parallel converter thread of the plurality of parallelconverter threads.

BRIEF DESCRIPTION OF THE DRAWING

These and other features, aspects, and advantages of the presentinvention will become better understood when the following detaileddescription is read with reference to the accompanying drawings in whichlike characters represent like parts throughout the drawings, wherein:

FIG. 1 illustrates a typical wind turbine generator power system fordelivering an electric power output through a power system converter toa load from the wind turbine generator;

FIG. 2 illustrates a block diagram of a typical power system employingmultiple parallel frequency and voltage converters;

FIG. 3 illustrates a block diagram of a typical thread one thread of apower converter system;

FIG. 4 illustrates common mode current flow in a power system converterwith paralleled converter threads;

FIG. 5 illustrates the power input and output connections for converterthreads of a power converter system being supplied by generator;

FIG. 6 illustrates an embodiment of a power system including a powerconverter system incorporating a plurality of parallel converter threadssupplied by isolated, wye-connected, power windings of a generator;

FIG. 7 illustrates another embodiment of a power system including apower converter system incorporating a plurality of parallel converterthreads supplied by isolated, delta-configured, power windings, of agenerator; and

FIG. 8 illustrates a flowchart for a method of eliminating common modecurrents in a power system supplying power output to a load from a powerconverter system including a plurality of parallel converter threads.

DETAILED DESCRIPTION OF THE INVENTION

The following embodiments of the present invention have many advantages,including eliminating the circulating common mode current betweenparallel converters threads. In order to eliminate the circulatingcommon mode current between parallel converters of the wind turbinesystem, the new structure and method described are to isolate outputs ofthe parallel converter threads on the load side by utilizing isolatedpower windings on a system power generator. Such an arrangementeliminates the need for a common mode inductor, promoting systemreliability and reducing the total system cost.

FIG. 5 illustrates the power output connections for prior art,non-isolated windings of a wind turbine generator power withgenerator-side parallel converter threads for a power converter system.The power system converter 20 includes parallel converter threads 20-1,20-2, 20-3, 20-n. The generator power windings 205 for wind turbinegenerator 14 may be configured in a wye arrangement, with power windings210, 211, 212 tied at the wye neutral point 215. The opposite end ofeach phase of the power windings 210, 211, 212 (opposite from the wyeneutral) may be tied to each input 250, 255, 260 of one correspondingphase (Phase A 230, Phase B 235, Phase C 240) for each of the parallelconverter threads 20-1 to 20-n. Inputs of corresponding phaseconnections from each of the converter threads are tied together Inputsfrom the converter threads 20-1 to 20-n are similarly tied at points270, 275 and 280 to converter side windings 25-1 for main transformer25, permitting common mode circulating current 205 to flow in closedloop between converter threads.

FIG. 6 illustrates an embodiment of a power system including a powerconverter system incorporating a plurality of parallel converter threadsbeing powered by isolated power windings of the generator in a wyeconfiguration. A wind turbine generator 14 operating under generatorcontroller 16 may supply inputs 340, 345, 350, 355 to an exemplary fourparallel converter threads 20-1 to 20-n of power system converter 20from four sets of isolated three-phase windings 310, 315, 320, 325 (oneisolated three-phase winding for each individual converter thread) ofgenerator power windings 305. In the instant example, isolated windingsin a wye configuration are illustrated. However, the isolated windturbine generator windings 405 may be configured in delta-configuredthree-phase windings 410, 415, 420 425 to supply input to the fourexemplary parallel converter threads 20-1 to 20-n of power systemconverter 20, as illustrated in FIG. 7. Although not illustrated, theisolated transformer windings for input from the generator to the powersystem converter may include a star configuration or any othertransformer arrangement suitable for power application. While thisembodiment has described a power system and a power converter for a windturbine generator, it is understood that the present invention may begenerally applicable to other types of power systems, electric powergenerators and power system converters.

Inputs for corresponding phases of converter threads 20-1 to 20-n to theconverter side windings 25-1 of main transformer 25 may remain tiedtogether at points 370 (470), 375 (475), 380, (480) as shown in FIG. 6(FIG. 7) respectively. Because the generator windings 305 (405) areisolated for the individual converter threads, the converter threads areno longer galvanically connected on the generator side, therebypreventing the flow of common mode currents on the generator sidebetween the converter threads. With the elimination of the common modecurrents flowing between the converter threads, the common mode reactormay be eliminated, reducing cost, complexity, and size of the converterthread.

Isolation of the generator windings for providing electric power inputto the converter may further be combined with isolation of the windingsfor the electric output of the parallel converter threads of theconverter on an output transformer.

In a further embodiment of the present invention, a method is providedfor eliminating common mode currents in a power system supplying a poweroutput to a load from a power converter system including a plurality ofparallel converter threads. The method may include operating a windturbine generator according to a wind turbine controller for providingan electrical output to a grid through a converter, including aplurality of parallel converter threads. The method also includessupplying electrical power to each of the parallel converter threadsthrough an isolated winding of the wind turbine generator that iselectrically isolated from the isolated winding supplying other threadsof the parallel converter. The method further includes operating theparallel converter threads to supply electrical power to the grid usingthe electrical power supplied by the isolated winding of the windturbine generator.

FIG. 8 illustrates a flowchart of a method for eliminating common modecurrents in a power system supplying a power output to a load from apower converter system including a plurality of parallel converterthreads. Initial step 510, operates a wind turbine generator accordingto a wind turbine generator controller for supplying an electricaloutput to a grid through a converter including a plurality of parallelconverter threads. Step 520 supplies electrical power to each of theparallel converter threads through an isolated winding of the windturbine generator. In step 530, the converter operates the parallelconverter threads to supply electrical power to the grid using theelectrical power supplied by the isolated winding of the wind turbinegenerator. In operating the parallel converter threads, the controllerfor the converter provides gating signals to the semiconductor switchesof the parallel converter threads. Gating by the parallel converterthreads may be performed in an interleaved or a non-interleaved pattern.

While various embodiments are described herein, it will be appreciatedfrom the specification that various combinations of elements, variationsor improvements therein may be made, and are within the scope of theinvention.

1. A power system for providing an output power to a load, the systemcomprising; a generator configured to generate an alternating currentinput power to a power converter system; a power converter systemcoupled to the generator and interconnected to generate an output powerand provide the output power to the load, wherein the converter systemincludes a plurality of parallel converter threads; a converter controlsystem coupled to the power converter system and configured to drive theconverter system to reduce harmonic components in the output power orthe alternating current input power; and isolation means for the inputpower to the converter from the generator, adapted to prevent commonmode current circulation between the parallel converter threads.
 2. Thepower system of claim 1, the power converter system further comprising:isolation means for the input power including a plurality of sets ofisolated power windings on the generator wherein each set of theplurality of sets of isolated power windings of the on the generatorinterconnects with one of a corresponding parallel converter thread ofthe plurality of parallel converter threads.
 3. The power system ofclaim 2 wherein the set of isolated power windings on the generatorcomprises a three-phase power winding and each of the plurality ofparallel converter threads comprises a three-phase power input.
 4. Thepower system of claim 3, wherein the three-phase power windingcomprises: wye-connected generator power output windings.
 5. The powersystem of claim 3 wherein the three-phase power winding comprises:delta-connected generator power output windings.
 6. The power system ofclaim 3, wherein the three-phase power winding comprises: star-connectedgenerator power output windings.
 7. The power system of claim 3, whereinthe generator comprises: a wind turbine generator.
 8. The power systemof claim 3, wherein the load comprises: an electric power grid.
 9. Thepower system of claim 3, wherein the converter control system drives theparallel converter threads of the converter system according to aninterleaved control scheme.
 10. The power system of claim 3, wherein theconverter control system drives the parallel converter threads of theconverter system according to a non-interleaved control scheme.
 11. Amethod for eliminating common mode currents in a power system supplyinga power output to a load from a power converter system including aplurality of parallel converter threads, the method comprising:operating the generator according to a controller to supply ac power toa power system converter; supplying the ac power to the power systemconverter through a plurality of sets of isolated power generatingwindings of the generator, wherein one set of the isolated powergenerator windings of the generator feeds a corresponding converterthread of the plurality of converter threads; operating the plurality ofconverter threads according to a converter control system for the powerconverter system adapted to drive the power converter system to reduceharmonic components in at least one the power output and the alternatingcurrent input power.
 12. The method for eliminating common mode currentsaccording to claim 11, comprising: supplying the ac power to the powersystem converter through sets of isolated three-phase power windings onthe generator.
 13. The method for eliminating common mode currentsaccording to claim 12, further comprising: supplying the ac power to thepower system converter through sets of isolated, wye-configured,three-phase power windings on the generator.
 14. The method foreliminating common mode currents according to claim 12, furthercomprising: supplying the ac power to the power system converter throughsets of isolated, delta-configured, three-phase power windings on thegenerator.
 15. The method for eliminating common mode currents accordingto claim 12, further comprising: supplying the ac power to the powersystem converter through sets of isolated, star-configured, three-phasepower windings on the generator.
 16. The method for eliminating commonmode currents according to claim 12, wherein the generator comprises awind turbine generator and the load comprises an electric power grid.17. The method for eliminating common mode currents according to claim12 further comprising: driving the parallel converter threads of theconverter system by the converter control system according to aninterleaved control scheme.
 18. The method for eliminating common modecurrents according to claim 12 further comprising: driving the parallelconverter threads of the converter system by the converter controlsystem according to a non-interleaved control scheme.
 19. A wind turbinepower system adapted to prevent common mode current circulation amongpower system converter threads when providing output power to anelectrical power grid, the system comprising; a wind turbine generatorcontrolled by a wind turbine controller configured to generate analternating current input power to a power converter system; a powerconverter system coupled to the wind turbine generator andinterconnected to generate an output power and provide the output powerto the load, wherein the power converter system includes a plurality ofthe parallel converter threads: a converter control system coupled tothe power converter system and configured to drive the parallelconverter threads to reduce harmonic components in the output power orthe alternating current input power; and a plurality of sets of isolatedpower windings on the wind turbine generator, wherein each set of theplurality of sets of isolated power windings of the generatorinterconnects with only one of a corresponding parallel converter threadof the plurality of parallel converter threads.
 20. The wind turbinepower system according to claim 18, wherein the sets of isolated powerwindings on the wind turbine generator comprise three-phase powerwinding configured in at least one of a wye configuration, a deltaconfiguration and a star configuration.